Nuclear pore complex ion channels (review)

Aging as a loss of morphostatic information: A developmental bioelectricity perspective

Maintaining order at the tissue level is crucial throughout the lifespan, as failure can lead to cancer and an accumulation of molecular and cellular disorders. Perhaps, the most consistent and pervasive result of these failures is aging, which is characterized by the progressive loss of function and decline in the ability to maintain anatomical homeostasis and reproduce. This leads to organ malfunction, diseases, and ultimately death. The traditional understanding of aging is that it is caused by the accumulation of molecular and cellular damage. In this article, we propose a complementary view of aging from the perspective of endogenous bioelectricity which has not yet been integrated into aging research. We propose a view of aging as a morphostasis defect, a loss of biophysical prepattern information, encoding anatomical setpoints used for dynamic tissue and organ homeostasis. We hypothesize that this is specifically driven by abrogation of the endogenous bioelectric signaling that normally harnesses individual cell behaviors toward the creation and upkeep of complex multicellular structures in vivo. Herein, we first describe bioelectricity as the physiological software of life, and then identify and discuss the links between bioelectricity and life extension strategies and age-related diseases. We develop a bridge between aging and regeneration via bioelectric signaling that suggests a research program for healthful longevity via morphoceuticals. Finally, we discuss the broader implications of the homologies between development, aging, cancer and regeneration and how morphoceuticals can be developed for aging.

Endogenous voltage gradients as mediators of cell-cell communication: strategies for investigating bioelectrical signals during pattern formation

Alongside the well-known chemical modes of cell-cell communication, we find an important and powerful system of bioelectrical signaling: changes in the resting voltage potential (Vmem) of the plasma membrane driven by ion channels, pumps and gap junctions. Slow Vmem changes in all cells serve as a highly conserved, information-bearing pathway that regulates cell proliferation, migration and differentiation. In embryonic and regenerative pattern formation and in the disorganization of neoplasia, bioelectrical cues serve as mediators of large-scale anatomical polarity, organ identity and positional information. Recent developments have resulted in tools that enable a high-resolution analysis of these biophysical signals and their linkage with upstream and downstream canonical genetic pathways. Here, we provide an overview for the study of bioelectric signaling, focusing on state-of-the-art approaches that use molecular physiology and developmental genetics to probe the roles of bioelectric events functionally. We highlight the logic, strategies and well-developed technologies that any group of researchers can employ to identify and dissect ionic signaling components in their own work and thus to help crack the bioelectric code. The dissection of bioelectric events as instructive signals enabling the orchestration of cell behaviors into large-scale coherent patterning programs will enrich on-going work in diverse areas of biology, as biophysical factors become incorporated into our systems-level understanding of cell interactions.

Receptors and ionic transporters in nuclear membranes: new targets for therapeutical pharmacological interventions

Work from our group and other laboratories showed that the nucleus could be considered as a cell within a cell. This is based on growing evidence of the presence and role of nuclear membrane G-protein coupled receptors and ionic transporters in the nuclear membranes of many cell types, including vascular endothelial cells, endocardial endothelial cells, vascular smooth muscle cells, cardiomyocytes, and hepatocytes. The nuclear membrane receptors were found to modulate the functioning of ionic transporters at the nuclear level, and thus contribute to regulation of nuclear ionic homeostasis. Nuclear membranes of the mentioned types of cells possess the same ionic transporters; however, the type of receptors is cell-type dependent. Regulation of cytosolic and nuclear ionic homeostasis was found to be dependent upon a tight crosstalk between receptors and ionic transporters of the plasma membranes and those of the nuclear membrane. This crosstalk seems to be the basis for excitation–contraction coupling, excitation–secretion coupling, and excitation – gene expression coupling. Further advancement in this field will certainly shed light on the role of nuclear membrane receptors and transporters in health and disease. This will in turn enable the successful design of a new class of drugs that specifically target such highly vital nuclear receptors and ionic transporters.

An epirubicin-conjugated nanocarrier with MRI function to overcome lethal multidrug-resistant bladder cancer

Multidrug resistance (MDR) presents a major obstacle to curing cancer. Chemotherapy failure can occur through both cell membrane drug resistance (CMDR) and nuclear drug resistance (NDR), and anticancer effectiveness of chemotherapeutic agents is especially reduced by their nuclear export. Here we report an exciting magnetically-targeted nanomedicine formed by conjugation of epirubicin (EPI) to non-toxic and high-magnetization nanocarrier (HMNC). Strikingly, HMNC-EPI overcomes both CMDR and NDR in human bladder cancer cell models, without using P-glycoprotein (P-gp) and nuclear pore inhibitors. Besides, the half-life of drug is prolonged ~1.8-fold (from 45 h to 81 h) at 37 °C, with a ~10-fold increase in concentration at the tumor site through magnetic targeting (MT). Moreover, malignant NDR bladder cancer can be effectively inhibited after 14 days in mice by just two injections and MT. We are the first to demonstrate the nanomedical strategy that can overcome the CMDR and NDR bladder cancers simultaneously, and proceed to the excellent MT therapy, significantly reducing the dosage and cardiotoxicity and holding great promise for incurable human MDR bladder cancer.

Nuclear membrane receptors and channels as targets for drug development in cardiovascular diseases

The use of confocal microscopy has shown that the nucleus plays an important role in excitation-contraction and excitation-secretion coupling of several excitable and nonexcitable cardiovascular cells. It has shown that the nuclear membranes, like the sarcolemmal membrane, possess ionic transporters as well as G protein-coupled receptors (GPCRs), which play a major role in modulating both cytosolic and nuclear ionic homeostasis and nuclear signalling. During spontaneous contraction of heart cells, the increase in cytosolic Ca2+ was immediately followed by a transient increase in nuclear Ca2+. The nuclear Ca2+ rise during excitation-contraction and excitation-secretion coupling was both dependent and independent of changes in cytosolic Ca2+. Nuclear membrane GPCRs, such as those of angiotensin II, neuropeptide Y, and ET-1, were functional and contributed to modulation of nuclear ionic homeostasis via direct and (or) indirect modulation of nuclear membrane ionic transporters such as channels, pumps, and exchangers. The signalling of nuclear membrane GPCRs may also contribute to modulation of gene expression, which may regulate proliferation and remodelling of cells and, indeed, life and death. Direct or indirect targeting of nuclear membrane ionic transporters and GPCRs may constitute a new target for drug action.

The direct effect of nuclear pores on nuclear chemotherapeutic concentration in multidrug resistant bladder cancer: the nuclear sparing phenomenon

PURPOSE

Multidrug resistance commonly limits effectiveness in treating malignancy with chemotherapy. Multidrug resistance has classically been described as a cell membrane phenomenon. Multidrug resistant cells are known to specifically exclude chemotherapy from the nucleus, resulting in lower nuclear concentrations than in the cytoplasm. This phenomenon is known as nuclear sparing and little is known of its etiology. We hypothesized that a component of the nuclear membrane, the nuclear pore, is responsible for this phenomenon.

MATERIALS AND METHODS

In this in vitro study we used the drug sensitive urothelial cancer cell line Massachusetts General Hospital urothelial 1 sensitive and its multidrug resistant subline Massachusetts General Hospital urothelial 1 resistant. After quantitative assessment of nuclear pores resistant and sensitive cells were fused using polyethylene glycol and laser scanning confocal microscopy was used to identify if drug resistant and sensitive nuclei can coexist within the same cell. The effect of inhibiting nuclear pore function using the specific pore inhibitor, wheat germ agglutinin, was assessed in whole cells using confocal microscopy and cytotoxicity assay as well as in isolated nuclei.

RESULTS

Nuclear pores appeared more numerous in multidrug resistance cells. Cell fusion experiments showed that multidrug resistance and sensitive nuclei could coexist with the same cell milieu. Wheat germ agglutinin reversed multidrug resistance in whole cells and isolated nuclei.

CONCLUSIONS

Multidrug resistance is a complex phenomenon occurring at many cellular levels, of which all may be potential therapeutic targets. The nuclear pore is involved in this process, which is to our knowledge a previously undescribed phenomenon. These experiments suggest that it may act to export drug from the nucleus, which is a process inhibited by wheat germ agglutinin.

Current concepts in nuclear pore electrophysiology

Over 4 decades ago, microelectrode studies of in situ nuclei showed that, under certain conditions, the nuclear envelope (NE) behaves as a barrier opposing the nucleocytoplasmic flow of physiological ions. As the nuclear pore complexes (NPCs) of the NE are the only pathways for direct nucleocytoplasmic flow, those experiments implied that the NPCs are capable of restricting ion flow. These early studies validated electrophysiology as a useful approach to quantify some of the mechanisms by which NPCs mediate gene activity and expression. Since electron microscopy (EM) and other non-electrophysiological investigations, showed that the NPC lumen is a nanochannel, the opinion prevailed that the NPC could not oppose the flow of ions and, therefore, that electrophysiological observations resulted from technical artifacts. Consequently, the initial enthusiasm with nuclear electrophysiology faded out in less than a decade. In 1990, nuclear electrophysiology was revisited with patch-clamp, the most powerful electrophysiological technique to date. Patch-clamp has consistently demonstrated that the NE has intrinsic ion channel activity. Direct demonstrations of the NPC on-off ion channel gating behavior were published for artificial conditions in 1995 and for intact living nuclei in 2002. This on-off switching/gating behavior can be interpreted in terms of a metastable energy barrier. In the hope of advancing nuclear electrophysiology, and to complement the other papers contained in this special issue of the journal, here I review some of the main technical, experimental, and theoretical issues of the field, with special focus on NPCs.

G-protein-coupled receptors, channels, and Na+–H+exchanger in nuclear membranes of heart, hepatic, vascular endothelial, and smooth muscle cellsThis paper is one of a selection of papers published in this Special Issue, entitled The Nucleus: A Cell Within A Cell

The action of several peptides and drugs is thought to be primarily dependent on their interactions with specific cell surface G-protein-coupled receptors and ionic transporters such as channels and exchangers. Recent development of 3-D confocal microscopy allowed several laboratories, including ours, to identify and study the localization of receptors, channels, and exchangers at the transcellular level of several cell types. Using this technique, we demonstrated in the nuclei of several types of cells the presence of Ca2+channels as well as Na+–H+exchanger and receptors such as endothelin-1 and angiotensin II receptors. Stimulation of these nuclear membrane G-protein-coupled receptors induced an increase of nuclear Ca2+. Our results suggest that, similar to the plasma membrane, nuclear membranes possess channels, exchangers and receptors such as those for endothelin-1 and angiotensin II, and that the nucleus seems to be a cell within a cell. This article will emphasize these findings.

Ca(2+)/CaM-dependent kinases: from activation to function

Calmodulin (CaM) is an essential protein that serves as a ubiquitous intracellular receptor for Ca(2+). The Ca(2+)/CaM complex initiates a plethora of signaling cascades that culminate in alteration of cellular functions. Among the many Ca(2+)/CaM-binding proteins to be discovered, the multifunctional protein kinases CaMKI, II, and IV play pivotal roles. Our review focuses on this class of CaM kinases to illustrate the structural and biochemical basis for Ca(2+)/CaM interaction with and regulation of its target enzymes. Gene transcription has been chosen as the functional endpoint to illustrate the recent advances in Ca(2+)/CaM-mediated signal transduction mechanisms.

Electrical dimension of the nuclear envelope

Eukaryotic chromosomes are confined to the nucleus, which is separated from the rest of the cell by two concentric membranes known as the nuclear envelope (NE). The NE is punctuated by holes known as nuclear pore complexes (NPCs), which provide the main pathway for transport of cellular material across the nuclear-cytoplasmic boundary. The single NPC is a complicated octameric structure containing more than 100 proteins called nucleoporins. NPCs function as transport machineries for inorganic ions and macromolecules. The most prominent feature of an individual NPC is a large central channel, ~7 nm in width and 50 nm in length. NPCs exhibit high morphological and functional plasticity, adjusting shape to function. Macromolecules ranging from 1 to >100 kDa travel through the central channel into (and out of) the nucleoplasm. Inorganic ions have additional pathways for communication between cytosol and nucleus. NE can turn from a simple sieve that separates two compartments by a given pore size to a smart barrier that adjusts its permeabiltiy to the metabolic demands of the cell. Early microelectrode work characterizes the NE as a membrane barrier of highly variable permeability, indicating that NPCs are under regulatory control. Electrical voltage across the NE is explained as the result of electrical charge separation due to selective barrier permeability and unequal distribution of charged macromolecules across the NE. Patch-clamp work discovers NE ion channel activity associated with NPC function. From comparison of early microelectrode work with patch-clamp data and late results obtained by the nuclear hourglass technique, it is concluded that NPCs are well-controlled supramolecular structures that mediate transport of macromolecules and small ions by separate physical pathways, the large central channel and the small peripheral channels, respectively. Electrical properties of the two pathways are still unclear but could have great impact on the understanding of signal transfer across NE and gene expression.

Calcium, ATP and nuclear pore channel gating

Nuclear envelope (NE) cisternal Ca2+ and cytosolic ATP are required for nuclear-pore-complex-(NPC-) mediated transport of DNAs, RNAs, transcription factors and other large molecules. Isolated cardiomyocyte nuclei, capable of macromolecular transport (MMT), have intrinsic NPC ion channel behavior. The large ion conductance (gamma) activity of the NPC channel (NPCC) is blocked by the NPC monoclonal antibody mAb414, known to block MMT, and is also silenced during periods of MMT. In cardiomyocytes, neither cytosolic Ca2+ nor ATP alone directly affects NPCC gating. To test the role of Ca2+ and ATP in NPCC activity, we carried out the present patch-clamp study with the pipette attached to the outer NE membrane of nuclei isolated from cultured Dunning G prostate cancer cells. Our investigations demonstrate that in these isolated nuclei neither cytosolic Ca2+ nor ATP alone directly affects NPCC gating. However, when simultaneously applied to the bath and pipette, they transiently silence NPCC activity through stimulation of MMT by raising the Ca2+ concentration in the NE cisterna ([Ca2+]NE). Our fluorescence microscopy observations with nuclear-targeted macromolecular fluorochromes (B-phycoerythrin and plasmid for the enhanced green fluorescence protein EGFP, pEGFP-C1) and with FITC-labeled RNA support the view that channel silence accompanies MMT. Repeated Ca2+ loading of the NE with Ca2+ and ATP, after unloading with 1-5 microM inositol 1,4,5-trisphosphate (IP3), thapsigargin (TSG) or 5 mM BAPTA or EGTA, failed to affect channel gating. This result indicates that other factors are involved in this phenomenon and that they are exhausted during the first cycle of NE Ca2+ loading/unloading--in agreement with current theories of NPC-mediated MMT. The results explain how Ca2+ and IP3 waves may convert the NE into an effective Ca2+ barrier and, consequently, affect the regulation of gene activity and expression through their feedback on MMT and NPCC gating. Thus, [Ca2+]NE regulation by intracellular messengers is an effective mechanism for synchronizing gene activity and expression to the cellular rhythm.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

Wheatgerm agglutinin-mediated toxicity in pancreatic cancer cells

Lectin binding specificities for carbohydrate allow phenotypic and functional characterization of membrane-associated glycoproteins expressed on cancer cells. This analysis examined wheatgerm agglutinin binding to pancreatic cancer cells in vitro and the resulting toxicity. Membrane preparations of nine human pancreatic carcinoma cell lines were studied for lectin binding using wheatgerm agglutinin (WGA), concanavalin A (ConA) and phytohaemagglutinin-L (PHA-L) in a lectin blot analysis. Cell proliferation in vitro was measured by thymidine incorporation in the absence or presence of lectins at various concentrations. Sialic acid binding lectins or succinyl-WGA (succWGA) served as controls. WGA toxicity was tested after swainsonine or neuraminidase pretreatment. Binding and uptake of fluorescein-labelled lectins was studied under fluorescence microscopy. All pancreatic cell lines displayed high WGA membrane binding, primarily to sialic acid residues. Other lectins were bound with weak to moderate intensity only. Lectin toxicity corresponded to membrane binding intensity, and was profound in case of WGA (ID50 at 2.5-5 microg ml(-1)). WGA exposure induced chromatin condensation, nuclear fragmentation and DNA release consistent with apoptosis. Important steps for WGA toxicity included binding to sialic acid on swainsonine-sensitive carbohydrate and lectin internalization. There was rapid cellular uptake and subsequent nuclear relocalization of WGA. In contradistinction to the other lectins studied, WGA proved highly toxic to human pancreatic carcinoma cells in vitro. WGA binding to sialic acid residues of N-linked carbohydrate, cellular uptake and subsequent affinity to N-acetyl glucosamine appear to be necessary steps. Further analysis of this mechanism of profound toxicity may provide insight relevant to the treatment of pancreatic cancer.

A nuclear localization signal can enhance both the nuclear transport and expression of 1 kb DNA

Although the entry of DNA into the nucleus is a crucial step of non-viral gene delivery, fundamental features of this transport process have remained unexplored. This study analyzed the effect of linear double stranded DNA size on its passive diffusion, its active transport and its NLS-assisted transport. The size limit for passive diffusion was found to be between 200 and 310 bp. DNA of 310–1500 bp entered the nuclei of digitonin treated cells in the absence of cytosolic extract by an active transport process. Both the size limit and the intensity of DNA nuclear transport could be increased by the attachment of strong nuclear localization signals. Conjugation of a 900 bp expression cassette to nuclear localization signals increased both its nuclear entry and expression in microinjected, living cells.

A Ca2+- and voltage-dependent cation channel in the nuclear envelope of red beet

The patch-clamp technique was applied to study ion conductances in various configurations of the nuclear envelope of non-enzyme-treated red beet (Beta vulgaris L.) nuclei. With excised patches a non-selective cation channel was observed, that was activated by micromolar concentrations of Ca2+ on the nucleoplasmic side of the envelope. The channel activity was also voltage-dependent and the voltage threshold of channel activation changed with the nucleoplasmic Ca2+ concentration. The most prominent conductance level was 110+/-22 pS with 150 mM KCl in the bath and pipette. The channel was permeable to small cations: permeabilities relative to K+ were PK congruent with PNa=1, PCs=0.3, but PCl=0.09. Calcium ions also permeated the channel with PCa=0.43, estimated from reversal potential, or 0.14, estimated from conductance ratio. Zn2+ (1 mM) when applied to the cytoplasmic side of the envelope blocked the channel activity completely, while amiloride (2 mM) reduced the channel current by 86% from the nucleoplasmic side. The properties of the whole-nucleus current (voltage-, time- and Ca2+-dependence) paralleled those observed with excised patches. The channel may provide a Ca2+-regulated pathway for passive diffusion of cations across the nuclear envelope and thus may play an important role in Ca2+-dependent nuclear processes ranging from gene transcription to apoptosis.

Patch-clamp detection of macromolecular translocation along nuclear pores

The present paper reviews the application of patch-clamp principles to the detection and measurement of macromolecular translocation along the nuclear pores. We demonstrate that the tight-seal 'gigaseal' between the pipette tip and the nuclear membrane is possible in the presence of fully operational nuclear pores. We show that the ability to form a gigaseal in nucleus-attached configurations does not mean that only the activity of channels from the outer membrane of the nuclear envelope can be detected. Instead, we show that, in the presence of fully operational nuclear pores, it is likely that the large-conductance ion channel activity recorded derives from the nuclear pores. We conclude the technical section with the suggestion that the best way to demonstrate that the nuclear pores are responsible for ion channel activity is by showing with fluorescence microscopy the nuclear translocation of ions and small molecules and the exclusion of the same from the cisterna enclosed by the two membranes of the envelope. Since transcription factors and mRNAs, two major groups of nuclear macromolecules, use nuclear pores to enter and exit the nucleus and play essential roles in the control of gene activity and expression, this review should be useful to cell and molecular biologists interested in understanding how patch-clamp can be used to quantitate the translocation of such macromolecules into and out of the nucleus.

Current evidence suggests independent regulation of nuclear calcium

We review and present current evidence supporting independent regulation of nuclear Ca2+ ([Ca2+]n). The nucleus and nuclear envelope contain proteins to both regulate and respond to changes in [Ca2+]n. However, this does not prove that [Ca2+]n is independently regulated from cytosolic Ca2+ ([Ca2+]c). Studies using fluorescent dyes suggested that changes in [Ca2+]n differed in magnitude from changes in [Ca2+]c. These studies have been criticised as the nuclear environment alters the fluorescent characteristics of these dyes. We have evaluated this question with aequorin targeted to the nucleus and cytoplasm and shown that the characteristics of the indicators are not altered in their respective environments. We have demonstrated that different stimuli induce changes in [Ca2+]n and [Ca2+]c that vary both temporally and in magnitude. The nucleus appeared to be shielded from increases in [Ca2+]c, either through a mechanism involving the nuclear envelope or by cytosolic buffering of localised increases in Ca2+. In addition, agonist stimulation resulted in an increase in [Ca2+]n, consistent with release from the perinuclear Ca2+ store. There was a stimulus dependence of the relation between [Ca2+]n and [Ca2+]c suggesting differential regulation of [Ca2+]n. These results have important implications for the role of Ca2+ as a specific regulator of nuclear events through Ca2+ binding proteins. In addition, they highlight the advantages of using targeted aequorin in intact cells to monitor changes in organelle [Ca2+].

Presence of zinc and calcium permeant channels in the inner membrane of the nuclear envelope

The nuclear envelope possesses specific ion channels that regulate the ionic traffic between the cytoplasm or the perinuclear space and the nucleoplasm. Using the patch-clamp technique to isolated rat nuclei exhibiting only the inner membrane of the nuclear envelope, we report the existence of calcium and zinc permeant channels. These channels displayed similar characteristics (conductance : 8 and 11 pS respectively, open time constant (3.5 ms and 3.7 ms) and close time constant (5.1 ms and 4.8 ms)) and were insensitive to different types of calcium channels blockers and to calcium concentration in the bathing solution. The exact role of these channels remains to define, but they may contribute to the regulation of intranuclear Ca++ or Zn++ dependent processes as important as cell proliferation or programmed cell death. Moreover, this work demonstrates that our nuclei preparation provides a way to study the inner membrane of the nuclear envelope.

Differential regulation of nuclear and cytosolic Ca2+ in HeLa cells

The results reported in this study address the controversial issue that nuclear free Ca2+ ([Ca2+]n) may be regulated independently of cytosolic free Ca2+ ([Ca2+]c). We have measured [Ca2+]n and [Ca2+]c with recombinant aequorin targeted to the nucleus and cytosol in HeLa cells. We found that histamine, ATP, and ionomycin increased [Ca2+]c quantitatively more than [Ca2+]n, although the time course of these changes was similar. The difference between [Ca2+]c and [Ca2+]n depended on the stimulus, and the relative difference between [Ca2+]n and [Ca2+]c was less with ionomycin than with histamine or ATP. After depletion of the internal Ca2+ store, restoration of extracellular Ca2+ resulted in only increased [Ca2+]c without a significant increase in [Ca2+]n. Treatment with cyclopiazonic acid resulted in a delayed increases in [Ca2+]n compared to [Ca2+]c. These differences in both timing and magnitude of nuclear Ca2+ signals confirm that the cell can limit or delay increases in nuclear free Ca2+. Taken with the fact that an inositol phosphate signaling system resides in the nucleus and its envelope, our data support the hypothesis that [Ca2+]n may be independently regulated.

Atomic force microscopy visualizes ATP-dependent dissociation of multimeric TATA-binding protein before translocation into the cell nucleus

The TATA-binding protein (TBP) is a universal transcription factor which plays an essential role in eukaryotic gene expression. As a karyophilic molecule, this cytosolic protein reaches its DNA-binding site through the transport channel of the nuclear pore complex. As occurs with other major cellular proteins, TBP forms multimers in solution, which is a limiting factor for nuclear translocation. While studying the nuclear translocation of TBP, we detected ATP-dependent multimerization of TBP with atomic force microscopy. In physiological solutions containing ATP, 14-molecule multimers dissociated into four-molecule multimers with a half-maximum dissociation constant of 10 microM. Electrophysiological experiments using isolated cell nuclei of cultured kidney cells revealed that TBP translocates into the cell nucleus only in the presence of ATP. When ATP was replaced with its slowly hydrolysing analogue, ATP[gamma-S] [i.e. adenosine 5'-o-(3-thiotriphosphate)], the aggregates remained intact and nuclear translocation was not possible. Taken together, our investigations suggest that TBP exhibits ATPase activity similar to that observed in relation to molecular chaperons. This activity secures physiological translocation of the transcription factor into the nucleus.

The ion channel behavior of the nuclear pore complex

Macromolecule-conducting pores have been recently recognized as a distinct class of ion channels. The poor role of macromolecules as electrical charge carriers can be used to detect their movement along electrolyte-filled pores. Because of their negligible contribution to electrical ion currents, translocating macromolecules reduce the net conductivity of the medium inside the pore, thus decreasing the measured pore ion conductance. In the extreme case, a large translocating macromolecule can interrupt ion flow along the pore lumen, reflected as a negligible pore conductance. Therefore, ion conductance serves as a measurement of macromolecular transport, with lesser values indicating greater macromolecular translocation (in size and/or number). Such is the principle of operation of the widely used Coulter counter, an instrument for counting and sizing particles. It has long been known that macromolecules translocate across the central channel of nuclear pore complexes (NPCs). Recently, large conductance ion channel activity (100-1000 pS) was recorded from the nuclear envelope (NE) of various preparations and it was suggested that NPCs may be the source of this activity. Despite its significance to understanding the regulation of transcription, replication, mRNA export, and thus gene expression of normal and pathological states, no report has appeared demonstrating that this channel activity corresponds to ion flow along the central channel of the NPC. Here we present such a demonstration in adult mouse cardiac myocyte nuclei. In agreement with concepts introduced for macromolecule-conducting channels, our patch clamp experiments showed that ion conductance is reduced, and thus that ion flow is restricted during translocation of macromolecules containing nuclear targeting signals. Ion flow was blocked by mAb414, a monoclonal antibody raised against a major NPC glycoprotein and known to localize on the NPC channel where it blocks macromolecular transport. These results also establish patch clamp as a useful technique for the measurement of macromolecular translocation along the large central channel of the NPC and provide a basis for the design of future investigations of nuclear signaling for control of gene activity, mRNA export for gene expression, as well as other processes subservient to NPC-mediated nucleocytoplasmic exchange.

Patch clamp and atomic force microscopy demonstrate TATA-binding protein (TBP) interactions with the nuclear pore complex

The universal TATA-binding protein, TBP, is an essential component of the multiprotein complex known as transcription factor IID (TFIID). This complex, which consists of TBP and TBP-associated factors (TAFs), is essential for RNA polymerase II-mediated transcription. The molecular size of human TBP (37.7 kD) is close to the passive diffusion limit along the transport channel of the nuclear pore complex (NPC). Therefore, the possibility exists that NPCs restrict TBP translocation to the nuclear interior. Here we show for the first time, with patch-clamp and atomic force microscopy (AFM), that NPCs regulate TBP movement into the nucleus and that TBP (10(-15)-10(-10)M) is capable of modifying NPC structure and function. The translocation of TBP was ATP-dependent and could be detected as a transient plugging of the NPC channels, with a concomitant transient reduction in single NPC channel conductance, gamma, to a negligible value. NPC unplugging was accompanied by permanent channel opening at concentrations greater than 250 pM. AFM images demonstrated that the TBP molecules attached to and accumulated on the NPC cytosolic side. NPC channel activity could be recorded for more than 48 hr. These observations suggest that three novel functions of TBP are: to stabilize NPC, to force the NPC channels into an open state, and to increase the number of functional channels. Since TBP is a major component of transcription, our observations are relevant to the understanding of the gene expression mechanisms underlying normal and pathological cell structure and function.

Patch clamp detection of transcription factor translocation along the nuclear pore complex channel

Transcription factors (TFs) are cytoplasmic proteins that play an essential role in gene expression. These proteins form multimers and this phenomenon is thought to be one of the mechanisms that regulate transcription. TF molecules reach their DNA binding sites through the large central channel of the nuclear pore complex (NPC). However, the NPC channel is known to restrict the translocation of molecules > or = 20-70 kD. Therefore, during their translocation, TF molecules and/or their multimers may plug the NPC channel and thus, interrupt ion flow through the channel, with a concomitant reduction in the ion conductance of the channel (gamma). Here we show with patch clamp that gamma is reduced during translocation of three major TFs: c-Jun (40 kD), NF-kappa B (approximately equal to 50 kD), and SP1 (approximately equal to 100 kD). Within a minute, femtomolar concentrations of these proteins reduced gamma suggesting a purely mechanical interaction between single TF molecules and the inner wall of the NPC channel. NPCs remained plugged for 0.5-3 hr in the absence of ATP but when ATP was added, channel plugging was shortened to < 5 min. After unplugging, channel closures were rarely observed and the number of functional channels increased. The transcription factors also stabilized the NPCs as shown by the extended duration of the preparations which allowed recordings for up to 72 hr. These observations are the first direct demonstration of the important role of NPCs in mediating nuclear translocation of TFs and, therefore, in forming part of the mechanisms regulating gene expression.(ABSTRACT TRUNCATED AT 250 WORDS)